Such cables must be capable of withstanding certain external stress forces exerted substantially radially relative to the longitudinal axis of the cable. These external stress forces lead to the cable sheath being squashed, thereby tending to compact the optical fiber ribbons and consequently to alter the optical transmission properties of the optical fibers, in particular by increasing the attenuation thereof, which is not desirable.
To increase the ability of optical transmission cables using ribbons to resist being squashed, proposals have already been made to house the optical fiber ribbons in a relatively rigid prefabricated structure having a plurality of slots, each ribbon being housed in a slot of the structure. However, that solution increases the manufacturing cost of cables. Also, the prefabricated structure tends to stiffen the cable, thereby giving rise to problems with cable laying.
In another known solution, provision is made for a cable sheath that is relatively rigid, and of relatively large diameter so as to leave sufficient empty space inside the sheath around the ribbons. As a result, if the sheath is deformed by being squashed, the ribbons can be pressed together in the space that remains free within the sheath. However that solution is not satisfactory since it goes against the need to reduce the section of optical transmission cables.